Picture it--you're in a Wal-Mart. Screaming 2-year old, apparently won't shut up and the mother of the kid is unable or unwilling to do anything to stop it. (This article recounts the incident). A 61-year old stranger then proceeds to slap the kid silly, and apparently this worked because (according to the police report) the grown man claimed "I told you I'd shut her up". The man was of course arrested, and the Internets are full of comments which range from (a) aghast parents who boast how they would have beaten the man to death if he had laid a finger on their wonderful little snowflakes to (b) cheering childless commenters who say the old man was a hero and his only mistake was not beating up the mother of the crying child.
Now, let's get a few points out of the way:
1) You can't go around hitting other people's kids, unless the kid hit you, and even then your response should be proportionate to their attack and their size. Even then, you need to give the parent a chance to discipline the kid themselves. But you can't hit them just for being annoying.
2) As a parent, you have a duty to others to keep your kids behaved. Obviously, a two year old is going to cry and scream when you bring them in public, which is a good reason to avoid bringing them in public until they're old enough to behave. When you do have to bring them in public, and you can't keep them from crying, you have to take them outside when possible. (Obviously, can't do this on airplanes! Frankly, don't take small kids on airplanes. Kids should either be moved by car or stay put until they're old enough to understand simple commands like SHUT THE HELL UP YOU LITTLE MONSTERS!!!). Restaurants? Limit it to Chuck E Cheese or some other kid restaurant, not fancy places (if you can afford to drop $100 on dinner, you can afford a baby sitter). And don't even get me started on movie theaters showing grown up films.
3) That said, I'm generally sympathetic to parents with crying babies because I know it is no picnic for them and while my discomfort will be over once they leave my sight, the parent has to deal with the kid for years. One thing I find works well at getting other people's kids to stop crying is to start crying yourself! The kid is understandably confused, and their own empathy kicks in, and they stop crying. Bingo! Of course then I run the risk of being slapped by a 61-year old stranger.
4) It was Wal-Mart. The old guy should have been glad the kid wasn't pooping in the aisle. And the mother should have been glad the old man didn't eat the kid.